Would there be much of a performance hit, routing issues, or corruption if a freely placed sidewalk overlaps the sidewalk of a road like this:

I've tested it in game and sims don't seem to have an issue with it. But then again, I'm only one person who's tested this in a very controlled environment over a stupidly short (for testing) span of time, so my data is no good. Has anyone played for any decant length of time in worlds where this was done and, if so, were there any issues that arose because of this? Is doing something like this in a very small part of the world still a terrible idea?

In CAW the routing lines look good I guess... but, to be honest, I don't know what bad would even look like. Here's a screen cap of that if that helps anyone more knowledgeable:

So, is this workable (aside from the lack of intersection caps) or something that should be avoided?

I don't think anything would happen, as the routing for the sidewalk is already there in the invisible sidewalk. The only issue is, that I can see, is if you specifically told the sim to go to that sidewalk, it might be confused as to which sidewalk. But then, I would think that it would decide on it's own and go there! I could see some weird behavior at the ends, though. Say, you end that overlay sidewalk and let it just continue as a normal one, you may have the sim walk oddly toward or away from the road, then back to the sidewalk on that transition.
By the way, why do you do this? If you want to put a sidewalk on the side of the road, why not just make it on the road itself? Any time I use opacity on the sidewalk is if I either don't want any sidewalk (rural roads) or if I want to have a grass/dirt strip for trees and poles between the road and sidewalk, at which point the sidewalk is simply placed parallel to the road, next to the invisible sidewalk (not over it).
Sorry for not being of more help.

I would never do it as I know roads on roads supposedly can create major issues and lots on roads, but others can advise more. As said above, why do the overlap when you can just use a visible sidewalk with the road you are using?

hhmmmmmm, can think of different uses after doing this (side without sidewalk with clif or canal) or as mentioned for a double road with green strip in between... But I know of issues when using to much roads (once made a square out of sidewalk and the sims walked over it like a labyrinth, funny but impractical

maybe you can fix that by using impassable terrain paint BUT you have to be very precise when applying it: to much and your road won't work either